A billfold can clearly be a gadget. We’ve covered many here on Gadget Lab, from a recycled bicycle inner-tube to the ridiculous carbon-fiber pocket-safe. But for plain good looks and an almost ridiculous commitment to simple design, the *Woolrich John Ric
Genius slipper design where you knit a short "scarf", fold and sew onto your bottom. Garter stitch extra chunky yarn about wide, and however many rows you need, then sew into the 2 layers of the felt bottom.